1. Field of the Invention
The present invention relates to charging devices used in electrophotographic image forming devices such as copying machines and printers for electrically charging surfaces of electrostatic latent image support members.
2. Description of the Related Art
There have been proposed charging devices of a type in which a surface of a photosensitive member, i.e., electrostatic latent image supporting member is charged by applying a voltage to a charging member such as a brush, roller or rotary endless belt which is in contact with or minutely spaced from the surface of the photosensitive member. Such charging devices have attracted the attention in view of the fact that they generates extremely small amount of ozone, which adversely affects surfaces of photosensitive members and human bodies, compared with corona charging devices.
However, pinholes or the similar defects in which the photosensitive layer is lost are likely to generate in the surface of the photosensitive member for the electrophotographic processing during the manufacturing process. In the charging device in which the charging member subjected to the high voltage directly contacts the surface of the photosensitive member or is faced thereto with a minute space as described above, the charging member and a substrate of the photosensitive member may be short-circuited through the portion not bearing the photosensitive layer. When short-circuited, the charging member is heated and may burn, resulting in the spread of fire to portions of the charging member other than the short-circuited portion in the extreme case. The burning generates unpleasant smoke, and the spread of fire impairs and renders the charging member inoperable, resulting in an extremely dangerous state.
In order to solve this problem, for example, U.S. Pat. No. 5,012,282 has disclosed a contact charging device in which a constant-current power supply is connected to a brush charging device, and a constant current is supplied to a photosensitive member through a brush formed of electrically conductive fibers. The constant current power supply can prevents the flow of an overcurrent to the charging member of the charging device even if the charging member encounters with the pinhole in the photosensitive member, and thus can prevent the burning or the like thereof. This can be achieved by the fact that, as shown in FIG. 9, the current i is constant regardless of variation of the load, i.e., existence and nonexistence of pinholes A, B, C, D as well as relative sizes thereof (e.g., A&lt;B&lt;C&lt;D). However, the applied charging voltage (a predetermined value is -1.2 kV in the illustrated example) varies in accordance with variation of load, so that charged potentials on the surface of the photosensitive member disadvantageously becomes irregular.
In practice, the load also changes due to history (residual potential) of the photosensitive member and irregularity of a photosensitive layer thickness of the photosensitive member. All kinds of variation of the load affects the image.